Luminaire for use with tubular fluorescent lamps



Sept. 18, 1945. w. J. STRA-SSBURGER ET AL 2,385,040

LUMINAIRE FOR USE WITH TUBULAR FLUORESCENT LAMPS Filed Dec. 18, 1940 3 Sheets-Sheet l Sept. 18, 1945. w. J. STRASSBURGER ET AL 2,385,040

LUMINAIRE FOR USE WITH TUBULAR FLUORESCENT LAMPS 1940 3 Sheets-Sheet 2 Filed Dec l8 MVENTORS p 1945- w. J. STRASSBURGER ETAL 2,385,040

LUMINAIRE FOR USE WITH TUBULAR FLUORESCENT LAMPS Filed Dec. 18, 1940 I5 Sheets-Sheet 3 m m a y 1 (A 2 3 i Q 2 .1 S 2 -IH Z i Q 2 2 x g Patented Sept. 18, 1945 LUMINAIRE FOR USE WITH TUBULAR FLUORESCENT LAMPS William J. Strassburger and Verne W. Hunter,

Pittsburgh, Pa., assignors to Grant Building, Incorporated, a corporation of Pennsylvania Application December 18, 1940, Serial No. 370,594

9 Claims.

This invention relates to fluorescent lighting and more particularly to ceiling fixtures for use with tulbular fluorescent lamps.

Where fluorescent lamps are to be used for office, store and similar illumination where appearance is important, the problem involved by their use not only includes obtaining the desired amount of illumination at given points within the room or enclosure, but also the distribution of the illumination throughout the enclosure and the appearance occasioned by such illumination. In other words, the problem involves light distribution which is substantially uniform throughout the enclosure, avoids objectionable glare from the light source itself and avoids sharp contrasts which approximate or have the appearance of shadows or dark areas within the enclosure or upon the walls of the enclosure.

Where the fluorescent lamps are to be used for ofiice or similar lighting, their position within the lighted enclosure is, of course, of importance. Experiments have disclosed that they are more efiectively used under conditions such that the ceiling of the room or enclosure contributes to the light distribution (by reflection. With this fact established, it then becomes apparent that the lamps are most effectively employed where they are supported from the ceiling. The problem then involves not only the relationship of the lamp (luminous tube) to a glare-preventing cover employed in connection with such lamp, but also the positioning of the lamp with relation to the supporting ceiling and such opaque parts or elements as must be employed for the purpose of permanently and efiectively mounting the lamp, its shielding cover and other auxiliary apparatus, in place on the ceiling of the room or enclosure.

In view of the above, an object of this invention is to produce a ceiling fixture for use with tubular type'fiuorescent lamps whereby substantially uniform glareless illumination may be obtained not only directly below the fixture but on the ceiling as well, including the ceiling area immediately adjacent the sides of the fixture.

Another object is to produce a simple rugged ceil ng luminaire for use with tulbular fluorescent lamps which is adapted to receive wiring and other lamp auxiliaries and accessories.

Another object is to produce a ceiling luminaire which is adapted to receive a fluorescent lamp and which is provided with a curved. troughlike, light-diffusing cover so located and of such form and density that the ratio of brightness of the light-diffusing cover and the adjacent ceiling areas are reduced to an acceptable value from the standpoint of both appearance and visual comfort.

Another object is to produce a ceiling luminaire of simple and rugged construction which adapted to receive a plurality of fluorescent lamps proved luminaire of sectional construction which is adapted to be assembled in varying lengths to accommodate varying numbers of fluorescent lamps arranged in end-to-end relation and which includes simple and effective means for mounting the lamps and for mounting light-emitting covers over the lamps.

These and other objects are attained by means of the fixture or luminaire described herein and illustrated in the drawings accompanying and forming a part hereof.

In the drawings:

Figure 1 is a fragmentary isometric view of a luminaire embodying this invention, the view being taken from a position such as to disclose the upper portion of the luminaire, i. e., that portion which engages a ceiling when the luminaire is so mounted.

Figure 2 is a fragmentary longitudinal sectional view of a. multiple-unit fixture or luminaire embodying the invention and shown mounted on a ceiling and with the fluorescent lamps and the light-emitting covers in place thereon. The section is taken along a plane which extends axially of the lamps, although the lamps and their supporting socket member are not shown in section.

Figure 3 is a transverse sectional view taken along the line III-III of Figure 2 and looking in the direction indicated by the arrows associated with the line III-III.

Figure 4 is a diagrammatic view of a troughlike base or runner, a fluorescent lamp, a lightemitting cover for the lamp and a light reflector and is for the purpose of defining the principle employed in positioning the lamp with relation to the shadow-casting edges of the' base or runner.

Figure 5 is a fragmental perspective view of a reflector constituting a part of the fixture here illustrated.

Figure 6 is a side elevation of a multiple-unit fixture or luminaire embodying this invention.

Figure '7 is a plan view (looking upwardly) of the fixture illustrated in Figure 6.

Figure 8 is a fragmental isometric view of the base or runner constituting a part of a fixture.

Y in place thereon.

The view corresponds in positioning with that of Figure 1.

The luminaire here illustrated is primarily adapted for ceiling mounting and the further description contemplates such a mounting. The principal structural features of the luminaire are so designed that they may be employed in the assembly of a single or a multiple-unit fixture. These parts are also so designed that the resulting fixtures are adapted to receive lamps of a standard size, viz., a 40 watt 48" fluorescent tubular type lamp and lamp covers of uniform length, although the invention is in no sense limited by the size of lamps or covers.

As illustrated, each luminaire consists essentially of a base or runner or lamp support, a fluorescent lamp, a light-emittingcover for the lamp, a reflector which extends longitudinally of the associated lamp and which, with the base, forms a housing for wiring and other lamp auxiliaries.

As shown, each base is made up of a base member 2!] and side members 2! so assembled as to produce a, channel-shaped r trough-like runner. For this purpose, the member is provided with an upwardly extending flange 23 along each longitudinal edge and the side members M are secured to these flanges in any suitable man-'- ner, for example, by spot welding. As shown, the side members 2| are so secured to the flanges 23 that they form supporting edges for the runner, i. e., edges which engage the supporting wall or ceiling and position the runner with relation thereto.

Each base member 20 is so formed, with relation to its flanges 23 that it is spaced from the ceiling or supporting wall when the runner is This spacing is sumcient to provide for such parts as securing nuts or bolts and also for the purpose of preventing the base from contacting the supporting wall except along its edges. The base member 20 forms a support for two angle brackets 27 which are oppositely positioned at the ends of the base member and each of which constitutes a support for a lampreceiving socket member. Each bracket 27 is roughly Z-shaped in cross-section and is bolted to the base member 20 as shown at is. The free arm of each Z-shaped bracket is designated by the numeral 34 and forms a support for a. reflector and also for a lamp-supporting socket member.

In Figure 2, one such socket member is designated by the numeral 29 and is secured in place on one of the brackets 27 by means of a bolt 29a. The opposed but cooperating socket member is designated by the numeral 30 in Figure 2 and is there illustrated as so formed as to provide a support for a lamp starter 3i. It is secured to its supporting bracket 21 by a bolt 29a. The lamp starter3i carried by the socket member 30 is cylindrical in shape and projects upwardly through a circular aperture 38 formed in the portio 34 of the associated bracket 27.

A reflector 32 bridges the space between and is supported at its ends by each pair of spaced brackets 27. Each reflector is in the form of a shallow V throughout the major portion of its length. However, the ends are partially flattened, aS shown at 35 in Figures 2 and 5, for the purpose of seating on the portion 3 3 of each supporting bracket 27. Each reflector is secured to its supporting bracket by means of screws 35 which extend through apertures 36 formed in the reflector, as shown in Figure 5. As there shown, each end of each reflector is provided with a slot 3'! of sufiicient size to accommodate the passage of a lamp starter iii and both ends of the reflector are so slotted for the purpose of making the reflectors interchangeable end to end.

Each reflector is preferably provided with a flange 32a along each of its longitudinal edges and these flanges are adapted to fit between adjacent portions of the side members 2! of the base or runner and in this way form a closure which is bounded by the side members 2i and the base member 20. Each base member is slotted, as at 24, at points along its length and these slots are adapted to receive mounting screws or bolts for the purpose of securing the runner to a supporting ceiling or wall. As shown in Figure 2, the mounting bolts are in the form of toggle bolts 25 and suspend the base member from the ceiling with its lateral edges in engagement with the ceiling. As previously noted, the intermediate portion of the base is spaced from its supporting wall with the result that a tightening up on the supporting bolts 25 will place the base member 20 under cross-bending strain and thus contribute to a situation which will tend to loci: the bolts against turning.

From the foregoing, it is apparent that a base member 2i], side members 2| and a reflector 32 may be assembled to constitute a single lamp support and in Figure 2, a lamp 5! is illustrated as supported by the opposed socket members 29 and 30. It will also be noted that the reflector is so mounted that the apex of its divergent reflecting faces points toward the lamp, extends longitudinally thereof and is in close proximity thereto throughout substantially the entire length of the lamp.

Where the fixture is adapted to receive a single lamp, then each end of the fixture will be provided with a cap or end member 39 which flts over the built-up base or runner and forms a closure for the end thereof. The cap may be secured to the base member in any suitable manner as, for example, by means of screws :89.

In Figure 2, we have illustrated the parts assembled to form a multiple-unit fixture and as there shown a connector 50 is employed for the purpose of securing the ends of aligned runners together. Each connector 40 has the double function of securing the separate runners together and of also cooperatin in supporting a lighttransmitting cover 4| which constitutes a part of each unit section of the luminaire.

As shown in the drawings, each lamp cover M is generally U-shaped in cross-section, is preferably of a standard length and is carried by the stirrup-shaped connectors 30 or a corresponding portion of a cap member 39, where a cap member is employed as a part of the cover support in the assembled unit. Each lamp cover is, to some extent, positioned by edge-receiving channels 22 formed on the side members 2! constituting a part of each runner or base section As shown, each channel 22 is formed as a reentrant flange on each base section and is located along the lower edge of each side member 2!. Where the side member is formed of sheet or strip metal, this reentrant flange may be formed by bending or otherwise shaping the metal.

The channels 22 of each pair of side members 2! cooperate in positioning a lamp cover M even though each cover is actually supported by a cap 39 and a connector 40 or by two connectors Gil. During the operation of locating a lamp cover -61 in place and looking it rigidly in position on the fixture, at least one of each of the cooperating cover support members (39 or 40) is provided with a pressure spring 45 which is adapted to engage one end of the cover and force it in a direction longitudinally of itself. For this reason, the cap 39 of Figure 2 is shown provided with a pressure spring 45 which is looped at a point intermediate its ends and is secured to a lug 48 by means of a screw 48a. The lug forms a part of the cap 39 and is located at the apex of its arch adjacent to the lowermost portion thereof. The opposite ends of the pressure spring 45 are shown bent back upon themselves so as to provide an extended cover-engaging portion for each spring. The spring ends engage the end of a lamp cover 4| and thus yieldingly hold the cover in place on the fixture. a

In Figure 2 of the drawings, one lamp cover 4| is shown forced by its associated pressure spring 45 against a rib 44 forming a part of a connector 40. In the drawings, a soft bufierfor example, a felt strip 43--intervenes between the end of the lamp cover and the rib 44 for the purpose of preventing breaking'of the cover.-

Each connector 40 is adapted to support two lamp covers 4| and for this reason each connector is equipped with a pressurespring 45, which is adapted to press one of such lamp covers longitudinally of itself and against a rib 44 of an adlacent connector or against a corresponding part of a cap 39. For this purpose each rib 44 is provided with a lug 48' corresponding in position and function to the lug 48 of the cap 39 of Figure 2.

A step of the operation of mounting a lamp cover 4| in place in a luminaire is diagrammatically illustrated in Figure 6 of the drawings. There the dotted rectangle 4| indicates the lamp cover in an initial position in which it is held during the operation of mounting it in place in the fixture. One e-id of the cover is tilted upwardly and the ends of its longitudinal edges are introduced into the channels 22 of the associated side members 2|. In this portion the cover 4| is moved against the free ends of the spring 45, thus compressing the spring until the opposite end of the lamp cover is capable of being moved past the connector 40 and to a position such that its edges 42 contact with the channels 22 throu hout their length. When the cover is in this position, it is moved in the direction of the pressure of its cooperating spring 45 until it is stopped by the cooperating rib 44. The lamp cover is held in this position by the pressure of its associated spring 46 and is thus prevented from accidental removal from its supporting members.

A multiple-unit luminaire is illustrated in Figures 6 and 7 and the illustration assumes that the springs 45 constituting a part of each section of the luminaire are positioned so as to force their associated covers to the right. Under such conditions, the right-hand cap 39 may be provided with a stop or rib corresponding to the rib 44 of the connector 40 illustrated in Figure 2, or its spring 45 may be so positioned and adjusted as to form a stop for the lamp cover 4| projecting into it and thus provide a situation wherein the lamp cover is yieldingly held in place between two opposed pressure springs.

trated and previously described. In addition, it is the contemplation of this invention to produce a luminaire which accomplishes light distribution by a combination of direct and indirect lighting and in which the ratio of brightness of the light-emitting portion of the luminaire and the indirect or the reflecting surfaces is maintained at an acceptable value from the standpoint of both appearance and visual comfort. This featureof the invention involves the avoidance of dense shadows on the portions of the supporting wall adjacent the luminaire and involves a special positioning of the tubular fluorescent lamp with relation to shadow casting portions of the fixture. It also involves employing a translucent or lightdifl'using. cover for the lamp which is specially located with relation to the light-emitting sur- The contemplation of the present invention is to provide a luminaire which is of relatively simple construction, is in expensive, and which con tributes to the easy installation and removal of lamps and lamp covers and the ready access to the associated lamp auxiliaries. All this is accomplished by the structural features here illusand also fall outside of the aforementioned face of the lamp and also with relation to shadowcasting portions of the luminaire.

To accomplish the desired distribution of light on the supporting wall or ceiling, the luminaire is so formed that the extreme shadow-casting edges of the base, i. e., of the support for the tubular fluorescent lamp 5| and its light-emitting cover 4|, are spaced apart not more than three times the external diameter of the tube BI and the tube is so located with relation to these edges that its external surface is tangent to the equal sides of an isosceles triangle which is based on these edges, has a base length equal to the distance between said edges (approximately 3 diameters of the secticn that it lies almost wholly without the triangle, but is at'no point located a distance substantially greater than the diameter of the tube from the nearest light-emitting surface of the tube. It has also been found desirable to limit the altitude of the base or runner so that it is substantially equal to the external diameter of the fluorescent tube 5|.

In Figure 4, we have diagrammatically illustrated the relationship of parts just described: The extreme shadow-casting edges of the base or runner are indicated at a and b. The isosceles triangle abc is based on these edges and its altitude cd is approximately equal to half its base. The tubular lamp 5| is so located with relation to the shadow-casting edges a and b that it is tangent to the lines ac and chef the aforementioned triangle. In addition, the lamp cover 4| a, as illustrated in Figure 4, is such that it is located almost wholly without the triangle, but no portion of this cover is spaced from the extemal surface of the tube an amount which greatly exceeds the diameter of the tube. As a matter of fact, all portions of the cover 4|a, except those immediately adjacent to the upper edges thereof, are within a diameter distance of the tube 4| I isosceles triangle.

In addition to employing a trough-shaped light-diffusing cover of the shape generally illus trated in Figure 3 or Figure 4, we have found it desirable to employ a light-emitting cover having an external corrugated surface with longitudinally projecting ridges 52 located between the cor rugations and consequently on the outside surface of the cover. These ridges, like other portions of the surface of the cover, emit light and emit light in all directions; consequently, they not only contribute to the difiusion of light and the prevention of shadows or marked contrast on the supporting wall or ceiling, but they also aid in lighting up the external surface of the base or runner and thus improve the general appearance of the luminaire. For design purposes we have employed a corrugated surface on the base or runner, but the important feature of the external surface of the base or runner is that the major portion of this surface is so positioned that it receives light directly from some light-emitting portion of the light-diffusing cover M a. In addition to the general relationship of the parts described in contributing to the appearance and the effectiveness of the luminaire, the reflector 32 is so located with relation to the upper or less effective side of the fluorescent tube 5|, that practically all light emitted by the tube is projected toward the light-diffusing cover Ma.

While the shape of the cover 4|, illustrated in Figure 3, is somewhat different from the cover illustrated in Figure 4, nevertheless all of the light-emitting, light-transmitting and light-refiecting structures illustrated in Figure 3 are positioned as described in connection with Figure 4 and function as there described.

It will, of course, be understood that the dis tribution of light over'the surface of the supporting wall or ceiling will be dependent upon the location of and the spacing between the luminaires mounted on that wall or ceiling. The shape of the fixture and its adaptability of being assembled into a unitary structure in which a number of lamps are located in end-to-end relationship, contributes to locating a number of such unitary structures in spaced parallel relationship on a ceiling or supporting wall. The number of such units employed and the distance between them will, of course, depend upon the amount of light desired, but it will be apparent that luminaires can be so positioned and spaced as to uniformly light the entire supporting wall or ceiling and without occasioning undue glare or undesirable contrasts. It will, therefore, be apparent that luminaires embodying the invention here defined may be employed in such a way as to contribute to a highly desirable light distribution throughout the entire interior of a lighted enclosure and because such luminaires contribute to an effective combination of direct and indirect lighting.

It will be understood that for convenience of assembly and also for the sake of appearance, the corrugations on the external surface of lamp covers M do not extend throughout the length of those covers, but terminate short of the ends of the covers so as to provide plain mounting surfaces at the ends. It will be understood that the. component parts of the base, i. e., the base member 20. the side M, and the brackets 27, will be preferabl made of thin metal, such as sheet metal. The lamp cover will be made of some light-transmitting material, such as glass or modern plastics. In addition, as many of the parts of the luminaire as possible will be of standard length and so made that they are interchangeable end for end. This is for the purpose of minimizing the cost of installation but it will, of course, be apparent that special parts will have to be made so as to provide for units of different lengths.

While we have illustrated and described but one embodiment of our invention, it will be apparent that additions to and omissions from the structural features illustrated may be made and that changes in the shape, position and relationship of some of the parts may be made without departing from the spirit and scope of the invention.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is:

1. In an illuminating fixture, an elongated, box-like runner comprising a base member adapted to be secured to a supporting wall, side members secured to said base member converging toward each other and each having a wallengaging edge and a longitudinally extending, cover-positioning surface which is parallel to the longitudinal axis of the runner and terminates in a shadow casting edge and a reflector intervening between said side members; two lamp sockets carried by said. runner, each positioned thereon between said positioning surfaces and located in opposed spaced relationship to the other; a tubular fluorescent lamp carried by said sockets with its axis parallel to the axis of the runner; an open-ended, substantially U-shaped, light-emitting, cover embracing said lamp with its longitudinal edges overlapping and paralleling said cover-positioning surfaces and each such edge being located in close proximity with one such surface; and spaced'supports carried by the runner, engaging the ends of said cover and supporting said cover; said cover-positioning surfaces being so located that the shadow casting edges thereof are spaced apart a distance no greater than three times the external diameter of said fluorescent tube, said tube being located in such relation to such edges that its external surface is tangent to the equal sides of an isosceles triangle based on said shadow casting edges, and having a base length equal to the distance between said edges and an altitude which is at least equal to one and one-half times the .diameter of said tube, said light-emitting cover being so located that at least the major portion thereof is located wholly outside said triangle, but that no internal surface thereof is spaced from said tubular lamp a distance substantially greater than c the diameter thereof.

2. In an illuminating fixture. an elongated, open-ended, trough-like, light-emitting cover, supporting means for said cover including two separate aligned supports spaced apart a distance less than the overall length of said cover and each embracing and supporting one end thereof, an elongated light source disposed within the confines of said cover and extending longitudinally thereof, a stop carried by one such support limiting the longitudinal movement of said cover in one direction, the other support being constructed and arranged to provide sufficient clearance to permit longitudinal movement of the cover away from said stop in an amount necessary for the stop-engaging end of such cover to clear the stop-carrying support on outward movement of such end of said cover, said cover being substantially U-shape in cross section and substantially uniform from end to end whereby it may be moved longitudinally in said supporting means without being first moved laterally thereof and spring means mounted on the support opposed to the stop-carrying support and engaging the adjacent end edge of said cover at spaced points thereof on opposite sides of said light source andyieldingly resisting longitudinal movement of said cover in a direction away end relationship; supporting means for said covers including two end supports, each'overlying and embracing the outer end of an end cover of said row and constituting a terminal closure for said row, and at least one intermediate support aligned with said end supports and embracing and supporting adjacent ends of contiguous covera of said row; a separate, elongated light source disposed within the confines of each such cover of said row and extending longitudinally thereof and substantially coaxially therewith; each of such supports co-operating with the next adjacent support in supporting one of the covers of said row and each support being spaced from its co-operating support a distance less than the overall length of the supported cover; one of such supports of each co-operating pair being provided with a stop for engaging the adjacent end of the supported cover and limiting longitudinal movement thereof in one direction and the other or second support of each such co-operating pair being constructed and arranged to provide suflicient clearance to permit longitudinal movement of the supported cover in a direction away from such stop in an amount necessary for the stop engaging end thereof to clear the stop-carrying support on outward movement of such end of said cover; and spring means carried by the second support of each co-operating pair and engaging the adjacent end edge of the supported cover at spaced points thereof on opposite sides of the light source located within such cover and yieldlngly resisting longitudinal movement oi the supported cover in a direction away from such stop and normally urging such cover against such stop.

4. In an,illuminating fixture, an elongated boxlike runner adapted to be secured to a supporting wall and comprising a base member, side members secured thereto, extending parallel to and converging toward each other and each having a cover-positioning surface extending longitudinally of said runner and terminating in a longitudinally extending shadow-casting edge, and a reflector intervening between said side members, spaced from and extending substantially parallel to, and substantially throughout the length of said runner; two lamp sockets carried by said runner each positioned thereon substantially midway between said side members and located in opposed relationship to the other; a tubular lamp carried by said sockets with its axis substantially parallel to the shadow-casting edges of said side members; an elongated, substantially U-shaped, open ended, light-emitting cover embracing said lamp with its longitudinal edges located between said side members and each such edge overlapping and paralleling one of said shadow-casting edges and located in close proximity with one of said cover-positioning surfaces; and two spaced supports carried by said runner, each engaging and supporting one end of said cover; said side members being so positioned that the shadow-casting edges thereof are spaced apart a distance no greater than three times the external diameter of said cover, said tubular lamp being so located with relation to said shadow-casting edges that its external cylindrical surface is tangent to the equal sides of an isosceles triangle based on said shadow-casting edges, having a base length substantially equal to the distance between such edges and an altitude equal to at least one and one-half times the diameter of the external cylindrical surface of said lamp and said light-emitting cover being so located with relation to said lamp that at least the major portion thereof is wholly outside such triangle.

5. In an illuminating fixture, an elongated box-like runner adapted to be secured to a supporting wall and comprising a base member, side members secured thereto and extending longitudinally of the axis of said runner, each terminating in a shadow-casting edge which extends along and is substantially parallel to the axis of said runner, and a reflector located between said side members and spaced from said base member; two lamp sockets carried by said runner, each spaced from the other and located substantially midway between said side members; a tubular lamp carried by said sockets extending substantially parallel with the axis of said runner and disposed adjacent said reflector; an elongated, substantially U-shaped, open-ended, lightemitting cover extending longitudinally, of and straddling said lamp with its longitudinal edges located between said shadow-casting edges and with such edge overlapping and paralleling one, of said shadow-casting edges; and spaced supports carried by said runner, each engaging and supporting one end of said cover; said side members being so positioned that the shadow-casting edges thereof are spaced apart a distance no greater than three times the external diameter of said tubular lamp, said tubular lamp being so located with relation to said shadow-casting edges that its external cylindrical surface is tangent to the equal sides of an isosceles triangle based on said shadow-casting edges, having a base substantially equal in length to the distance between such edges and an altitude at least equal to one and one-half times the diameter of the external cylindrical surface of said lamp.

6. In an illuminating fixture, an elongated box-like runner adapted to be secured to a supporting wall and comprising an elongated base member, side members secured thereto and extending longitudinally thereof and a reflector spaced therefrom extending substantially parallel thereto throughout substantially the length thereof and located between said side members; each such side member terminating in a shadowcasting edge which is substantially parallel to the axis of said runner and having a cover-positioning surface adjacent such edge and extending substantially throughout the length thereof; two lamp sockets carried by said runner each spaced from the other and located substantially midway between said side members; a tubular lamp carried by said sockets with its axis substantially parallel to said shadow-casting edges and the adjacent surface of said reflector; a substantially U-shaped, light-emitting cover extending longitudinally of and'straudling said lamp and overlying said reflector with its longitudinal edges located between said side members and with each such edge overlapping and extending substantially parallel to the shadow-casting edge of one such member and extending in close proximity with one of said cover-positioning surfaces; spaced supports carried by said runner, each an gaging and supporting one end of said cover; said tubular lamp being so located with relation to said shadow-casting edges that its external cylindrical surface is tangent to the equal sides of an isosceles triangle based on said shadowcasting edges and having a base substantially equal in length to the distance between such edges and an altitude equal to at least one and onehalf times the diameter of the external cylindrical surface of said lamp and said light-emitting cover being so located with relation to said lamp that the major portion thereof is wholly outside such triangle.

'7. In an illuminatin fixture. an elongated box-=- like runner adapted to be secured to a ceiling and comprising a base member adapted to be located adjacent the supporting ceiling, downwardly pro= jecting side members and a reflector; each side member extending longitudinally of the base member, being secured thereto along one of its edges, terminating in a, shadow-casting edge and carrying a cover-positioning surface located adjacent said shadow-casting edge and between it and said base member, said reflector being spaced from and extending substantially parallel to said base member and intervening between the cover positioning surfaces of said side members; two lamp supporting sockets carried by said runner, located thereon in spaced opposed relationship to each other between said side members and projecting beyond the plane defined by the shadow-casting edges thereof; a tubular lamp carried by said sockets, located beyond the plane defined by said shadow-casting edges and extending substantially parallel to the axis of said run ner; an open-ended, trough-shaped, light-emit ting cover embracing said lamp with its axis substantially parallel to the axis thereof and with each of its longitudinal edges overlapping the shadow-casting edge of a side member and extending substantially parallel thereto and in close proximity with one of said cover-positioning sur faces; two stirrup-like supports carried by said runner, each engaging and supporting one end of said cover and each spaced from the other a distance less than the overall length of said cover; one such support bein provided with a stop for engaging one end of said cover and limiting lon gitudinal movement thereof in one direction, the other such support having sumoient clearance to permit longitudinal movement of said cover in a direction away from said stop in an amount neeessary for the stop-engaging end of said cover to clear the stop-carrying support on outward movement thereof,.said enclosure having a transverse section of substantially U-shape which is nniform throughout the length of said cover, where by the cover may be moved longitudinally in said supports without first being moved laterally thereof; and spring means, mounted on the sul port opposed to said stop-carrying support, and engaging the adjacent end edge of said cover at spaced points thereof on opposite sides of said light source and yieldingly resisting longitudinal movement of said cover in a direction away from said stop.

8. In an illuminating fixture, an elongated box-like runner secured to a ceiling and comprising an elongated base member located adja cent the supporting ceiling, side members secured to the lateral edges of said base member, and each having a ceiling-engaging edge and a shadow-casting edge substantially parallel to the axis of said base member throughout substan tially the length thereof, and a reflector spaced from said base member and located between said side members; two lamp supporting sockets carried by said runner, located thereon in spaced,

opposed relationship to each other between said side members and projecting below the plane de fined by the shadow-casting edges thereof; a tubular lamp carried by said sockets, located below the plane defined by said shadow-casting edges and extending substantially parallel to the aids oi aeeaoao said runner; a trough-shaped, light-emitting cover embracing said lamp. extending throughout the length thereof with its axis substantially parallel to the axis thereof and with its longitudinal edges located between said side members and each such edge overlapping the shadowcasting edge of a side member and extending substantiallyparallel thereto; two downwardly errtending supports carried by said runner, each engaging and supporting one end of said cover and each spaced from the other a distance less than the overall length of said cover; a stop carried by one of said supports for engaging one end of said cover and limiting the longitudinal movement thereof in one direction, the other such support providing sumcient clearance to permit longitudinal movement of said cover in a direction away from said stop in an amount necessary for the stop-engaging end of said cover to clear the stop-carrying support on outward movement thereof; said cover having a transverse section of substantially U-shape which is substantially uniform throughout the length of the cover, whereby the cover may be moved longitudinally of said supports without first being moved laterally thereof; and spring means mounted on the support, opposed to the stop-carrying support,

and engaging the adjacent end edge of said cover and yieldingly resisting longitudinal movement said cover away from said stop and normally urging said cover against said stop.

9. In a multiple unit illuminating fixture, an elongated box-like runner adapted to be mounted on a ceiling, comprising a base member, longitudinally extending side members secured to lateral edges thereof and extending parallel to each other and each projecting downwardly irom said base member and terminating in a shadow-casting edge; a row of trough-like, light-emittin covers spaced apart and disposed in aligned endtoend relationship; supporting means for said covers including two end supports, each embracing the outer end of an end cover and at least one intermediate support aligned with said end supports and embracing and supporting the ends of contiguous covers of the row, each of such supports co-operating with the next adjacent support in supporting one cover of saidrow and each such support Ming spaced from its co-operating support a distance less than the overall length of the supported cover; a separate tubular lamp car ried by said runner and disposed within the confines of each such cover of said row, each such a lamp extending longitudinally of such cover and substantitally co-axially therewith; a cover-engaging stop carried by one of each pair of cooperating supports for engagingthe adjacent end of the supported cover and limiting longitudinal movement thereof in one direction and the sec- 0nd or other support of each such co-operating pair having sufdcient clearance to permit the sup- I 

